1. Field of the Invention
The present invention relates to a wireless visible light communication system, and more particularly to an apparatus and method for transmitting information using a wavelength division multiplexing (WDM) scheme in a wireless visible light communication system.
2. Description of the Related Art
Korea's Ministry of Commerce, Industry and Energy has forecasted that current illuminations will be replaced with LED by the year 2015. Visible light communication refers to a system for wirelessly transmitting data through use of objects, such as light emitting diode (LED)-based indoor/outdoor illuminations, advertising boards, traffic lights, street lights and the like. Such a visible light communication system can be applied in such diverse locations as a hospital, an airplane, etc., where the use of radio frequency (RF) is limited, and can provide supplementary information service using an electric display board.
The Wavelength Division Multiplexing (WDM) technology refers to a technology that carries various data, received from different places, through a single optical fiber, and corresponds to a light transmission scheme for increasing the communication capacity and speed. Each piece of data is transmitted at a specific optical wavelength, where one optical fiber can carry up to 80 wavelengths or data channels. This means that the amount of transmission data to be transmitted through the conventional optical fiber can increase up to 80 times. According to the WDM technology, different types of data with different transmission rates, such as Internet data, synchronous optical network (SONET) data, and asynchronous transfer mode (ATM) data, can be transmitted together. There is an attempt to apply such a WDM transmission technology to a white LED. In the case of an RGB LED, which is a kind of white LED, it is possible to apply a wired WDM technology to wireless visible light communication in such a manner as to independently transmit pieces of data through each channel of Red, Green, and Blue light.
FIG. 1 is a view illustrating the configuration of a conventional visible light communication system and represents an application scheme of visible light communication, which is performed through visible light.
Referring to FIG. 1, an access point (AP) 100 including a plurality of LEDs transmits a visible light signal to a terminal. The AP 100 includes a plurality of LEDs 101, 102, and 103. The terminal may include a mobile terminal 120 and a fixed-type terminal 121. FIG. 1 shows a case where the AP performs both an illumination function and a communication function at the same time. In addition, a case of communication between terminals may be considered. When communication between terminals 130, 131, and 132 is performed through use of visible light, the user can select an object to communicate and can communicate with the object while seeing a communication-available range, so that the system has a function of providing high safety in terms of security, rather than an illumination function.
FIG. 2 is a block diagram illustrating the configuration of conventional WDM visible-light communication transmitter/receiver.
The visible light communication transmitter includes a plurality of encoders 210, 211, and 212 that are connected in parallel with each other and channel-code data to transmit; a plurality of modulators 220, 221, and 222 which are connected in parallel with each other and modulate data channel-coded by the encoders; at least one light generator for transmitting a signal, modulated by the modulators, as a visible light signal; and a controller 200 for controlling each component in the visible light communication transmitter.
The visible light communication receiver includes at least one light sensor for receiving a visible light signal; a plurality of demodulators 280, 281, and 282 that are connected in parallel with each other and demodulate a signal received from the light sensor; a plurality of decoders 290, 291, and 292 that are connected in parallel with each other and, receive a signal demodulated by the demodulators. The decoders perform a channel decoding operation on the demodulated signal so as to restore data; and a controller 270 for controlling each component in the visible light communication receiver. In the WDM visible-light communication apparatus, communication paths operate independently according to the path to transmit information has been determined.
FIG. 3 is a flowchart illustrating a conventional transmission/reception operation for WDM visible-light communication.
The visible light communication transmitter/receiver start their operations, the visible light communication transmitter transmits a visible light signal in step 301, and the visible light communication receiver receives the visible light signal from the visible light communication transmitter in step 303. In step 305, it is determined if the transmission/reception operations are completed. When it is determined that the transmission/reception operations are completed, the transmission/reception operations of the visible light communication are terminated. In contrast, when it is determined that the transmission/reception operations are not completed, the procedure returns to step 301 and the above steps are repeated until the transmission/reception operations are completed.
A color of light is determined according to the energy ratio among the three primary colors mixed together. That is, the energy distribution rate according to each wavelength of light determines the color of light. The relation between energy according to each wavelength of light and the color of light can be identified by making reference to a chromaticity diagram.
FIG. 4 is a view illustrating the energy distribution according to each wavelength when white light is made in the conventional scheme, wherein the energy distribution according to each wavelength is obtained when white light is made by generating light through use of a red LED, a blue LED, and a green LED, and mixing the generated light.
White light has energy existing over all wavelength bands of red, green, and blue, but the energy distribution shows different energy levels according to wavelengths. That is, the green energy level is lowest, and energy levels are higher in the sequence of blue and red.
The conventionally used WDM technology is usually used for optical fiber communication. In this case, there is no difference between energies allocated according to wavelengths of red, green, and blue, so that the same value is given to each independent channel, and the same energy are allocated to each wavelength.
In contrast, a white LED used for illumination includes different transmission energies depending on each wavelength band, as shown in FIG. 4. Therefore, when the WDM technology is applied under such an environment, the transmission energies of wavelengths become the same, so that the original function as an illumination is lost due to the imbalance of a color ratio. In order to solve such a problem, there is a request for a WDM technology that enables the greatest transmission capability to be obtained while controlling the energy ratio among RGB colors.